So traditionally I've often been a bit surprised at celebrities having these sort of scandals because they can probably have their pick of women anyway. As in I'm like, why do they need to force themselves on anyone, why risk going out with an underage girl (Adam Johnson etc - I appreciate there's no suggestion that Brand did so)? Russell Brand during this period was famous and regarded as good looking, women would lust after him, by his own admission he was very promiscuous. So when there's a queue of attractive women at your door I don't really get why he'd allegedly assault someone as it seems like a very high risk:reward ratio.
But I then sort of came to the conclusion that if you are wired to behave a certain way, be it through wanting to exert power over people or whatever, then I guess those urges still control your character. You feel you are taking what you deserve and you don't like the hit to the ego of rejection, perhaps magnified if you are used to getting your own way. As others have mentioned, he comes across as a bit of a narcissist.
When the story broke, rightly or wrongly I wasn't that surprised. I can picture him this way, right down the text messages afterwards saying sorry etc.